Plasma, or more generally, an electrical discharge, has found extensive use in a variety of industrial applications, including material processing. For example, during semiconductor processing, plasma is often utilized to assist etch processes to facilitate anisotropic removal of material along fine lines or within vias (or contacts) patterned on a semiconductor substrate. Examples of such plasma-assisted etching include reactive ion etching (“RIE”), which is in essence an ion activated chemical etching process.
During plasma processing, ion energy, and more specifically, the ion energy distribution (“IED”), is a process parameter that strongly influences the outcome of the reactive process at the substrate. For example, when performing an etching process on a semiconductor device, ion energy affects etch selectivity, etch rate uniformity, sidewall profile, residue control, etc. Due to the significance of this process parameter, the measurement of ion energy and its distribution at a specific location within a plasma processing system is important for characterizing the effectiveness of the plasma.
Generally, the IED is measured by immersing a grid and an ion collector within a beam of ions. The electric potential of the grid is varied such that only the ions in the beam having sufficient energy to overcome the potential barrier imposed by the biased grid will pass through the grid and strike the ion collector. By collecting and measuring the ion current as a function of the potential on the grid, an integrated form of the IED may be acquired. Differentiation of this integral leads to the IED.
While IED has been measured extensively in plasma processes for decades using a variety of ion energy analyzers (“IEA”), there remains needed improvement. For example, most known conventional analyzers perturb the processing plasma to an extent that the measurement is no longer characteristic of the conditions prevailing when processing a substrate, fail to operate at large electric potentials, and/or exhibit substantive noise arising from secondary electron emission within the analyzer.
While many attempts have been made to cure these shortcomings, there still remains the need for improved, novel, and practical solutions to these and other problems.